Cadmium oxide and cadmium hydroxide prepared through neutralization and formation treatment of cadmium nitrate have heretofore been predominantly used as the starting material of the negative active material of nickel-cadmium batteries.
The cadmium oxide or cadmium hydroxide, after being fixed through coating or impregnation therewith of a substrate, is usually reduced into metallic cadmium through partial charge thereof to an extent of about 30 to 40% of the capacitance of a negative electrode prior to the construction of the battery.
Since, however, such reduction of the cadmium oxide or cadmium hydroxide into metallic cadmium through partial charge of the former involves drawbacks in the operation such as the necessity of taking a series of complicated and troublesome steps of electrolysis, washing with water, drying, etc., there has been proposed preliminary addition of a metallic cadmium powder as part of the starting material of the negative active material with a view to obviating such drawbacks.
From necessity under such circumstances, there has been proposed the use of various metallic cadmium powders prepared as the negative active materials by respective known methods.
Examples of such metallic cadmium powders include one prepared by the atomization of molten cadmium with a high-pressure gas or liquid; one prepared by the evaporation and condensation method wherein molten cadmium is heated above the boiling point and then condensed in an oxygen-free cooler; one prepared by the so-called evaporation-in-gas method wherein metallic cadmium heated above the boiling point in a high-frequency induction furnace, a plasma furnace, a resistance furnace or the like is evaporated in an evaporator evacuated and then fed with argon gas, xenon gas, nitrogen gas, etc., alone or in combination, to adjust the internal pressure of the evaporator to a negative pressure of a few to hundreds of Torr, and is then stuck to the wall of the evaporator, a cooling plate or the like to be collected in the form of a metallic cadmium powder; one prepared by the electrolysis method; one prepared by the substitutive precipitation method; and one prepared through mechanical pulverization.
Each of the metallic cadmium powders respectively prepared by the foregoing methods is very fine but still too large in particle size to act as the active material of the battery, and hence is low in the utilization factor of the active material to make it difficult to put it into practical use. It is well known that a metallic cadmium powder to be used as the negative active material generally has the utilization factor of the active material affected by the shape and size of the particles thereof. More specifically, a metallic cadmium powder having an uneven surface provides a more excellent result as regards the utilization factor as the active material than one having a wide and configurationally simple smooth surface.
According to the above-mentioned evaporation and condensation method well known as one of the conventional methods of preparing a metal powder, the resulting metallic cadmium powder is as large as a few to about 10 .mu.m in particle diameter, and is in the form of nearly spherical particles each with a smooth surface resulting from the surface tension in a molten state. Such a spherical or tetragonal metallic cadmium powder has a small surface area because of the smooth surface, and hence is low in electrical activity to overcome the problem of a low utilization factor in discharge.
In order to increase the surface area of such a spherical particle, the diameter of the particle must be made as small as possible. According to the conventional evaporation and condensation method, however, cadmium is evaporated from molten metallic cadmium sufficiently heated and then formed into particles in a state of vigorous evaporation of cadmium wherein evaporated particles repeatedly collide and fuse with each other to form droplets by the action of the surface tension thereof. In such a stage, it is considerably difficult to control the particle diameter to be below a submicron level. Accordingly, the specific surface area of the particle is at best 0.2 to 0.6 m.sup.2 /g or smaller, and the utilization factor as the active material is as low as about 20 to 40% to render the particles useless as the active material of the battery from the practical point of view.
Further, metallic cadmium powders respectively prepared according to other methods, though as very fine as submicron or smaller particles, are obtained in very low yields to pose problems such as the utter industrial inapplicability of the methods, the high cost of the powders, and hence the difficulty of use thereof from an economical aspect despite the satisfactory utilization factors as the active materials.